Shared memory is the fastest interprocess communication mechanism. The
operating system maps a memory segment in the address space of several
processes, so that several processes can read and write in that memory
segment without calling operating system functions. However, we need some
kind of synchronization between processes that read and write shared memory.

Consider what happens when a server process wants to send an HTML file
to a client process that resides in the same machine using network mechanisms:

The server must read the file to memory and pass it to the network
functions, that copy that memory to the OS's internal memory.

The client uses the network functions to copy the data from the OS's
internal memory to its own memory.

As we can see, there are two copies, one from memory to the network and
another one from the network to memory. And those copies are made using
operating system calls that normally are expensive. Shared memory avoids
this overhead, but we need to synchronize both processes:

The server maps a shared memory in its address space and also gets
access to a synchronization mechanism. The server obtains exclusive
access to the memory using the synchronization mechanism and copies
the file to memory.

The client maps the shared memory in its address space. Waits until
the server releases the exclusive access and uses the data.

Using shared memory, we can avoid two data copies, but we have to synchronize
the access to the shared memory segment.

Request to the operating system a memory segment that can be shared
between processes. The user can create/destroy/open this memory using
a shared memory object: An
object that represents memory that can be mapped concurrently into
the address space of more than one process..

Associate a part of that memory or the whole memory with the address
space of the calling process. The operating system looks for a big
enough memory address range in the calling process' address space and
marks that address range as an special range. Changes in that address
range are automatically seen by other process that also have mapped
the same shared memory object.

Once the two steps have been successfully completed, the process can start
writing to and reading from the address space to send to and receive data
from other processes. Now, let's see how can we do this using Boost.Interprocess:

As we've mentioned we have to use the shared_memory_object
class to create, open and destroy shared memory segments that can be mapped
by several processes. We can specify the access mode of that shared memory
object (read only or read-write), just as if it was a file:

When a shared memory object is created, its size is 0. To set the size
of the shared memory, the user must use the truncate
function call, in a shared memory that has been opened with read-write
attributes:

shm_obj.truncate(10000);

As shared memory has kernel or filesystem persistence, the user must explicitly
destroy it. The remove
operation might fail returning false if the shared memory does not exist,
the file is open or the file is still memory mapped by other processes:

Once created or opened, a process just has to map the shared memory object
in the process' address space. The user can map the whole shared memory
or just part of it. The mapping process is done using the mapped_region class. The class represents
a memory region that has been mapped from a shared memory or from other
devices that have also mapping capabilities (for example, files). A mapped_region can be created from any
memory_mappable object
and as you might imagine, shared_memory_object
is a memory_mappable object:

usingboost::interprocess;std::size_tShmSize=...//Map the second half of the memory
mapped_regionregion(shm//Memory-mappable object
,read_write//Access mode
,ShmSize/2//Offset from the beginning of shm
,ShmSize-ShmSize/2//Length of the region
);//Get the address of the region
region.get_address();//Get the size of the region
region.get_size();

The user can specify the offset from the mappable object where the mapped
region should start and the size of the mapped region. If no offset or
size is specified, the whole mappable object (in this case, shared memory)
is mapped. If the offset is specified, but not the size, the mapped region
covers from the offset until the end of the mappable object.

Let's see a simple example of shared memory use. A server process creates
a shared memory object, maps it and initializes all the bytes to a value.
After that, a client process opens the shared memory, maps it, and checks
that the data is correctly initialized:

Windows operating systems provide shared memory using memory backed
by the paging file but the lifetime semantics are different from the
ones defined by POSIX (see Native
windows shared memory section for more information).

In those platforms, shared memory is emulated with mapped files created
in a "boost_interprocess" folder created in a temporary files
directory. In Windows platforms, if "Common AppData" key is present
in the registry, "boost_interprocess" folder is created in that
directory (in XP usually "C:\Documents and Settings\All Users\Application
Data" and in Vista "C:\ProgramData"). For Windows platforms
without that registry key and Unix systems, shared memory is created in
the system temporary files directory ("/tmp" or similar).

Because of this emulation, shared memory has filesystem lifetime in some
of those systems.

This function can fail if the shared memory
objects does not exist or it's opened by another process. Note that this
function is similar to the standard C intremove(constchar*path) function. In UNIX systems, shared_memory_object::remove calls shm_unlink:

The function will remove the name of the shared memory object named
by the string pointed to by name.

If one or more references to the shared memory object exist when is
unlinked, the name will be removed before the function returns, but
the removal of the memory object contents will be postponed until all
open and map references to the shared memory object have been removed.

Even if the object continues to exist after the last function call,
reuse of the name will subsequently cause the creation of a boost::interprocess::shared_memory_object
instance to behave as if no shared memory object of this name exists
(that is, trying to open an object with that name will fail and an
object of the same name can be created again).

In Windows operating systems, current version supports an usually acceptable
emulation of the UNIX unlink behaviour: the file is renamed with a random
name and marked as to be deleted when the last open handle is
closed.

Creating a shared memory segment and mapping it can be a bit tedious when
several processes are involved. When processes are related via fork()
operating system call in UNIX systems a simpler method is available using
anonymous shared memory.

This feature has been implemented in UNIX systems mapping the device \dev\zero
or just using the MAP_ANONYMOUS
in a POSIX conformant mmap
system call.

This feature is wrapped in Boost.Interprocess
using the anonymous_shared_memory() function, which returns a mapped_region object holding an anonymous
shared memory segment that can be shared by related processes.

Here is an example:

#include<boost/interprocess/anonymous_shared_memory.hpp>#include<boost/interprocess/mapped_region.hpp>#include<iostream>#include<cstring>intmain(){usingnamespaceboost::interprocess;try{//Create an anonymous shared memory segment with size 1000
mapped_regionregion(anonymous_shared_memory(1000));//Write all the memory to 1
std::memset(region.get_address(),1,region.get_size());//The segment is unmapped when "region" goes out of scope
}catch(interprocess_exception&ex){std::cout<<ex.what()<<std::endl;return1;}return0;}

Once the segment is created, a fork() call can be used so that region is used to communicate two related
processes.

Windows operating system also offers shared memory, but the lifetime of
this shared memory is very different to kernel or filesystem lifetime.
The shared memory is created backed by the pagefile and it's automatically
destroyed when the last process attached to the shared memory is destroyed.

Because of this reason, there is no effective way to simulate kernel or
filesystem persistence using native windows shared memory and Boost.Interprocess emulates shared memory using
memory mapped files. This assures portability between POSIX and Windows
operating systems.

However, accessing native windows shared memory is a common request of
Boost.Interprocess users because they
want to access to shared memory created with other process that don't use
Boost.Interprocess. In order to manage
the native windows shared memory Boost.Interprocess
offers the windows_shared_memory
class.

Windows shared memory creation is a bit different from portable shared
memory creation: the size of the segment must be specified when creating
the object and can't be specified through truncate
like with the shared memory object.

Take in care that when the last process attached to a shared memory is
destroyed the shared memory is destroyed
so there is no persistency with native
windows shared memory. Native windows shared memory has also another limitation:
a process can open and map the whole shared memory created by another process
but it can't know which is the size of that memory. This limitation is
imposed by the Windows API so the user must somehow transmit the size of
the segment to processes opening the segment.

Sharing memory between services and user applications is also different.
To share memory between services and user applications the name of the
shared memory must start with the global namespace prefix "Global\\". This global namespace
enables processes on multiple client sessions to communicate with a service
application. The server component can create the shared memory in the global
namespace. Then a client session can use the "Global" prefix
to open that memory.

The creation of a shared memory object in the global namespace from a session other than
session zero is a privileged operation.

Let's repeat the same example presented for the portable shared memory object:
A server process creates a
shared memory object, maps it and initializes all the bytes to a value. After that,
a client process opens the shared memory, maps it, and checks
that the data is correctly initialized. Take in care that if the server exits before
the client connects to the shared memory the client connection will fail, because
the shared memory segment is destroyed when no proces is attached to the memory.

In many UNIX systems, the OS offers another shared memory memory mechanism, XSI
(X/Open System Interfaces) shared memory segments, also known as "System V" shared memory.
This shared memory mechanism is quite popular and portable, and it's not based in file-mapping
semantics, but it uses special functions (shmget, shmat, shmdt, shmctl...).

Unlike POSIX shared memory segments, XSI shared memory segments are not identified by names but
by 'keys' usually created with ftok. XSI shared memory segments have kernel lifetime and
must be explicitly removed. XSI shared memory does not support copy-on-write and partial shared memory mapping
but it supports anonymous shared memory.

Let's repeat the same example presented for the portable shared memory object:
A server process creates a shared memory object, maps it and initializes all the bytes to a value. After that,
a client process opens the shared memory, maps it, and checks
that the data is correctly initialized.

File mapping is the association of a file's contents with a portion of the address space
of a process. The system creates a file mapping to associate the file and the address
space of the process. A mapped region is the portion of address space that the process
uses to access the file's contents. A single file mapping can have several mapped regions,
so that the user can associate parts of the file with the address space of the process
without mapping the entire file in the address space, since the file can be bigger
than the whole address space of the process (a 9GB DVD image file in a usual 32
bit systems). Processes read from and write to
the file using pointers, just like with dynamic memory. File mapping has the following
advantages:

Uniform resource use. Files and memory can be treated using the same functions.

Automatic file data synchronization and cache from the OS.

Reuse of C++ utilities (STL containers, algorithms) in files.

Shared memory between two or more applications.

Allows efficient work with a large files, without mapping the whole file into memory

If several processes use the same file mapping to create mapped regions of a file, each
process' views contain identical copies of the file on disk.

File mapping is not only used for interprocess communication, it can be used also to
simplify file usage, so the user does not need to use file-management functions to
write the file. The user just writes data to the process memory, and the operating
systems dumps the data to the file.

When two processes map the same file in memory, the memory that one process writes is
seen by another process, so memory mapped files can be used as an interprocess
communication mechanism. We can say that memory-mapped files offer the same interprocess
communication services as shared memory with the addition of filesystem persistence.
However, as the operating system has to synchronize the file contents with the memory
contents, memory-mapped files are not as fast as shared memory.

Create a mappable object that represent an already created file of the
filesystem. This object will be used to create multiple mapped regions of the
the file.

Associate the whole file or parts of the file with the address space of the
calling process. The operating system looks for a big enough memory address range
in the calling process' address space and marks that address range as an
special range. Changes in that address range are automatically seen
by other process that also have mapped the same file and those changes
are also transferred to the disk automatically.

Once the two steps have been successfully completed, the process can start writing to
and reading from the address space to send to and receive data from other processes
and synchronize the file's contents with the changes made to the mapped region.
Now, let's see how can we do this using Boost.Interprocess:

First, we have to link a file's contents with the process' address space. To do
this, we have to create a mappable object that represents that file. This is
achieved in Boost.Interprocess creating a file_mapping object:

After creating a file mapping, a process just has to map the shared memory in the
process' address space. The user can map the whole shared memory or just part of it.
The mapping process is done using the mapped_region class. as we have said before
The class represents a memory region that has been mapped from a shared memory or from other
devices that have also mapping capabilities:

usingboost::interprocess;std::size_tFileSize=...//Map the second half of the file
mapped_regionregion(m_file//Memory-mappable object
,read_write//Access mode
,FileSize/2//Offset from the beginning of shm
,FileSize-FileSize/2//Length of the region
);//Get the address of the region
region.get_address();//Get the size of the region
region.get_size();

The user can specify the offset from the file where the mapped region
should start and the size of the mapped region. If no offset or size is specified,
the whole file is mapped. If the offset is specified, but not the size,
the mapped region covers from the offset until the end of the file.

If several processes map the same file, and a process modifies a memory range
from a mapped region that is also mapped by other process, the changes are
inmedially visible to other processes. However, the file contents on disk are
not updated immediately, since that would hurt performance (writing to disk
is several times slower than writing to memory). If the user wants to make sure
that file's contents have been updated, it can flush a range from the view to disk.
When the function returns, the flushing process has startd but there is not guarantee that
all data has been written to disk:

//Flush the whole region
region.flush();//Flush from an offset until the end of the region
region.flush(offset);//Flush a memory range starting on an offset
region.flush(offset,size);

Remember that the offset is not an offset on the file, but an offset in the
mapped region. If a region covers the second half of a file and flushes the
whole region, only the half of the file is guaranteed to have been flushed.

Let's reproduce the same example described in the shared memory section, using
memory mapped files. A server process creates a shared
memory segment, maps it and initializes all the bytes to a value. After that,
a client process opens the shared memory, maps it, and checks
that the data is correctly initialized::

#include<boost/interprocess/file_mapping.hpp>#include<boost/interprocess/mapped_region.hpp>#include<iostream>#include<fstream>#include<string>#include<vector>#include<cstring>#include<cstddef>#include<cstdlib>intmain(intargc,char*argv[]){usingnamespaceboost::interprocess;//Define file names
constchar*FileName="file.bin";conststd::size_tFileSize=10000;if(argc==1){//Parent process executes this
{//Create a file
file_mapping::remove(FileName);std::filebuffbuf;fbuf.open(FileName,std::ios_base::in|std::ios_base::out|std::ios_base::trunc|std::ios_base::binary);//Set the size
fbuf.pubseekoff(FileSize-1,std::ios_base::beg);fbuf.sputc(0);}//Remove on exit
structfile_remove{file_remove(constchar*FileName):FileName_(FileName){}~file_remove(){file_mapping::remove(FileName_);}constchar*FileName_;}remover(FileName);//Create a file mapping
file_mappingm_file(FileName,read_write);//Map the whole file with read-write permissions in this process
mapped_regionregion(m_file,read_write);//Get the address of the mapped region
void*addr=region.get_address();std::size_tsize=region.get_size();//Write all the memory to 1
std::memset(addr,1,size);//Launch child process
std::strings(argv[0]);s+=" child ";if(0!=std::system(s.c_str()))return1;}else{//Child process executes this
{//Open the file mapping and map it as read-only
file_mappingm_file(FileName,read_only);mapped_regionregion(m_file,read_only);//Get the address of the mapped region
void*addr=region.get_address();std::size_tsize=region.get_size();//Check that memory was initialized to 1
constchar*mem=static_cast<char*>(addr);for(std::size_ti=0;i<size;++i)if(*mem++!=1)return1;//Error checking memory
}{//Now test it reading the file
std::filebuffbuf;fbuf.open(FileName,std::ios_base::in|std::ios_base::binary);//Read it to memory
std::vector<char>vect(FileSize,0);fbuf.sgetn(&vect[0],std::streamsize(vect.size()));//Check that memory was initialized to 1
constchar*mem=static_cast<char*>(&vect[0]);for(std::size_ti=0;i<FileSize;++i)if(*mem++!=1)return1;//Error checking memory
}}return0;}

As we have seen, both shared_memory_object and file_mapping objects can be used
to create mapped_region objects. A mapped region created from a shared memory
object or a file mapping are the same class and this has many advantages.

One can, for example, mix in STL containers mapped regions from shared memory
and memory mapped files. The libraries that only depend on mapped regions can
be used to work with shared memory or memory mapped files without recompiling them.

In the example we have seen, the file or shared memory contents are mapped
to the address space of the process, but the address was chosen by the operating
system.

If several processes map the same file/shared memory, the mapping address will be
surely different in each process. Since each process might have used its address space
in a different way (allocation of more or less dynamic memory, for example), there is
no guarantee that the file/shared memory is going to be mapped in the same address.

If two processes map the same object in different addresses, this invalids the use
of pointers in that memory, since the pointer (which is an absolute address) would
only make sense for the process that wrote it. The solution for this is to use offsets
(distance) between objects instead of pointers: If two objects are placed in the same
shared memory segment by one process, the address of each object will be different
in another process but the distance between them (in bytes) will be the same.

So the first advice when mapping shared memory and memory mapped files is to avoid
using raw pointers, unless you know what you are doing. Use offsets between data or
relative pointers to obtain pointer functionality when an object placed in a mapped
region wants to point to an object placed in the same mapped region. Boost.Interprocess
offers a smart pointer called boost::interprocess::offset_ptr that
can be safely placed in shared memory and that can be used to point to another
object placed in the same shared memory / memory mapped file.

The use of relative pointers is less efficient than using raw pointers, so if a user
can succeed mapping the same file or shared memory object in the same address in two
processes, using raw pointers can be a good idea.

To map an object in a fixed address, the user can specify that address in the
mappedregion's constructor:

mapped_regionregion(shm//Map shared memory
,read_write//Map it as read-write
,0//Map from offset 0
,0//Map until the end
,(void*)0x3F000000//Map it exactly there
);

However, the user can't map the region in any address, even if the address is not
being used. The offset parameter that marks the start of the mapping region
is also limited. These limitations are explained in the next section.

As mentioned, the user can't map the memory mappable object at any address and it can
specify the offset of the mappable object that is equivalent to the start of the mapping
region to an arbitrary value.
Most operating systems limit the mapping address and the offset of the mappable object
to a multiple of a value called page size. This is due to the fact that the
operating system performs mapping operations over whole pages.

If fixed mapping address is used, offset and address
parameters should be multiples of that value.
This value is, typically, 4KB or 8KB for 32 bit operating systems.

//These might fail because the offset is not a multiple of the page size
//and we are using fixed address mapping
mapped_regionregion1(shm//Map shared memory
,read_write//Map it as read-write
,1//Map from offset 1
,1//Map 1 byte
,(void*)0x3F000000//Aligned mapping address
);//These might fail because the address is not a multiple of the page size
mapped_regionregion2(shm//Map shared memory
,read_write//Map it as read-write
,0//Map from offset 0
,1//Map 1 byte
,(void*)0x3F000001//Not aligned mapping address
);

Since the operating system performs mapping operations over whole pages, specifying
a mapping size or offset that are not multiple of the page size will waste
more resources than necessary. If the user specifies the following 1 byte mapping:

//Map one byte of the shared memory object.
//A whole memory page will be used for this.
mapped_regionregion(shm//Map shared memory
,read_write//Map it as read-write
,0//Map from offset 0
,1//Map 1 byte
);

The operating system will reserve a whole page that will not be reused by any
other mapping so we are going to waste (page size - 1) bytes. If we want
to use efficiently operating system resources, we should create regions whose size
is a multiple of page size bytes. If the user specifies the following two
mapped regions for a file with which has 2*page_size bytes:

//Map the first quarter of the file
//This will use a whole page
mapped_regionregion1(shm//Map shared memory
,read_write//Map it as read-write
,0//Map from offset 0
,page_size/2//Map page_size/2 bytes
);//Map the rest of the file
//This will use a 2 pages
mapped_regionregion2(shm//Map shared memory
,read_write//Map it as read-write
,page_size/2//Map from offset 0
,3*page_size/2//Map the rest of the shared memory
);

In this example, a half of the page is wasted in the first mapping and another
half is wasted in the second because the offset is not a multiple of the
page size. The mapping with the minimum resource usage would be to map whole pages:

When two processes create a mapped region of the same mappable object, two processes
can communicate writing and reading that memory. A process could construct a C++ object
in that memory so that the second process can use it. However, a mapped region shared
by multiple processes, can't hold any C++ object, because not every class is ready
to be a process-shared object, specially, if the mapped region is mapped in different
address in each process.

When placing objects in a mapped region and mapping
that region in different address in every process,
raw pointers are a problem since they are only valid for the
process that placed them there. To solve this, Boost.Interprocess offers
a special smart pointer that can be used instead of a raw pointer.
So user classes containing raw pointers (or Boost smart pointers, that
internally own a raw pointer) can't be safely placed in a process shared
mapped region. These pointers must be replaced with offset pointers, and
these pointers must point only to objects placed in the same mapped region
if you want to use these shared objects from different processes.

Of course, a pointer placed in a mapped region shared between processes should
only point to an object of that mapped region. Otherwise, the pointer would
point to an address that it's only valid one process and other
processes may crash when accessing to that address.

References suffer from the same problem as pointers
(mainly because they are implemented as pointers).
However, it is not possible to create a fully workable
smart reference currently in C++ (for example,
operator.() can't be overloaded). Because of this,
if the user wants to put an object in shared memory,
the object can't have any (smart or not) reference
as a member.

References will only work if the mapped region is mapped in the same
base address in all processes sharing a memory segment.
Like pointers, a reference placed in a mapped region should only point
to an object of that mapped region.

The virtual table pointer and the virtual table
are in the address space of the process
that constructs the object, so if we place a class
with a virtual function or virtual base class, the virtual
pointer placed in shared memory will be invalid for other processes
and they will crash.

This problem is very difficult to solve, since each process needs a
different virtual table pointer and the object that contains that pointer
is shared across many processes. Even if we map the mapped region in
the same address in every process, the virtual table can be in a different
address in every process. To enable virtual functions for objects
shared between processes, deep compiler changes are needed and virtual
functions would suffer a performance hit. That's why
Boost.Interprocess does not have any plan to support virtual function
and virtual inheritance in mapped regions shared between processes.

Static members of classes are global objects shared by
all instances of the class. Because of this, static
members are implemented as global variables in processes.

When constructing a class with static members, each process
has its own copy of the static member, so updating a static
member in one process does not change the value of the static
member the another process. So be careful with these classes. Static
members are not dangerous if they are just constant variables initialized
when the process starts, but they don't change at all (for example,
when used like enums) and their value is the same for all processes.